The investigation into the White House leak of a CIA agent's identity is now focusing on whether two top administration officials provided misleading statements to the FBI, it was reported yesterday.

According to press accounts, Karl Rove, the president's chief political adviser, and Lewis "Scooter" Libby, the vice-president's chief of staff, both provided testimony that was later contradicted by other evidence.

The revelations come at a time when the burgeoning scandal over the outing of a CIA undercover agent, Valerie Plame, is threatening to engulf the White House.

Ms Plame was the wife of a government critic, Joseph Wilson, who had questioned the justification of the Iraq invasion, and the leak is alleged to have been an attempt to discredit or intimidate him.

In an initial round of interviews with investigators, Mr Rove is reported to have omitted to mention that he had discussed the agent's identity with a Time reporter in July 2003, a few days after Mr Wilson had published a highly critical article.

According to a Bloomberg news agency report, Mr Libby testified that he had first heard about Ms Plame from an NBC television journalist, Tim Russert. But according to NBC, Mr Russert denied the claim in his evidence to a grand jury last year.

A statement by NBC at the time said Mr Russert "did not know Ms Plame's name or that she was a CIA operative and that he did not provide that information to Mr Libby".

The New York Times yesterday reported that at the time of the leak, Mr Rove and Mr Libby had been collaborating on the administration's response to Mr Wilson's central allegation that President George Bush had misled the American public in his January 2003 State of the Union address.

In that speech, laying out the case for war, the president cited evidence of Iraqi attempts to buy uranium in Niger, west Africa. The claim was found to have been based on forged documents.

George Tenet, the director of the CIA at the time, took responsibility for the false claim, helping to draw fire away from the White House, but yesterday's report suggests that Mr Rove and Mr Libby had a role in drafting his public admission.

The news that the two senior officials were intimately involved in the issue added to scepticism about their claims to have initially heard about Ms Plame from journalists, rather than the other way round.

Leaking the identity of an undercover agent is a serious crime under US law, but prosecutors would have to prove that the leaker was aware of the agent's covert status. However, the investigation, led by federal prosecutor Patrick Fitzgerald, is reported also to be investigating possible charges of perjury and obstruction of justice.

Meanwhile, a parallel investigation is under way into who forged the Niger documents. They are known to have been passed to an Italian journalist by a former Italian defence intelligence officer, Rocco Martino, in October 2002, but their origins have remained a mystery. Mr Martino has insisted to the Italian press that he was "a tool used by someone for games much bigger than me", but has not specified who that might be.

A source familiar with the inquiry said investigators were examining whether former US intelligence agents may have been involved in possible collaboration with Iraqi exiles determined to prove that Saddam Hussein had a nuclear programme.